General description of procedure, equipment, technique

Aortic valve replacement for severe symptomatic aortic stenosis (AS) is a Class I ACC/AHA recommendation (Level of Evidence B). The surgical aortic valve replacement (SAVR) was the only available treatment option for severe AS until 2002, when Cribier et al described the first successful transcatheter aortic valve replacement (TAVR) with a balloon-expandable valve by an antegrade, transseptal approach through the femoral vein.

Since then, the procedure has evolved from an antegrade transvenous approach to a retrograde transarterial approach with a high success rate and has been recognized as a treatment option for inoperable patients with severe symptomatic AS. It is FDA approved for this purpose and carries an ESC/EACTS class I indication.

For high-risk patients, FDA approval is expected to be imminent and it carries an ESC/EACTS class IIa indication. Numerous retrospective studies and registries, as well as the randomized controlled Placement of Aortic Transcatheter Valves (PARTNER) trial have established the safety, feasibility, and efficacy of TAVR in inoperable and high-risk patients with severe symptomatic AS.

These results are of practical significance, since approximately one third of the patients with severe symptomatic AS are deemed ineligible for surgery due to the presence of comorbidities. At present, most of the available evidence on the feasibility, safety and efficacy of TAVR is derived from two transcatheter heart valve types: balloon-expandable Edwards SAPIEN and Sapien-XT valves (Edwards Lifesciences Corporation, Irvine, CA) and self-expanding Medtronic CoreValve (Medtronic Inc., Minneapolis, MN) (Figure 1).

Balloon-expandable valves

Cribier-Edwards valve was a 1st generation balloon-expandable valve and has been superseded by the next generation valves.

Edwards-SAPIEN valve is a bovine pericardium trileaflet valve mounted on a stainless steel frame and is available in 2 sizes: 23 mm for annulus size 18-22 mm (22 French delivery system) and 26 mm for annulus size 23-26 mm (24 French delivery system). Edwards-SAPIEN valve was used in the randomized, controlled PARTNER trial in patients with severe symptomatic AS and is presently the only FDA approved device.

Sapien-XT is the 3rd generation bovine pericardium trileaflet balloon-expandable valve mounted on a cobalt-chromium frame. It is widely used outside the United States and is currently being evaluated in the randomized, controlled PARTNER-2 trial, a pivotal FDA trial for safety and efficacy. The Sapien-XT valve is available in 4 sizes: 20-mm valve for annulus size 16-19 mm by 2D TEE (only commercially available in Japan at present with an 18 French delivery system), 23-mm valve for annulus size 18-22 mm (18 French delivery system), 26-mm valve for annulus size 21-25 mm (19 French delivery system), and 29 mm valve for annulus size 24-27 mm (22 French delivery system).

The balloon-expandable Edwards valves can be implanted by the transfemoral approach (preferred approach by most operators) or transapical approach in patients who are not candidates for transfemoral approach.

Self-expanding valves

Medtronic CoreValve is a porcine pericardial trileaflet valve with a self-expanding nitinol frame and is currently available outside the U.S. in three sizes: 26-mm valve for annulus size 20- 23 mm by 2D TEE, 29-mm valve for annulus size 23-27 mm, and 31-mm valve for annulus size 26-29 mm. The Medtronic CoreValve is implanted using an 18 French delivery system.

Medtronic CoreValve can be implanted by the transfemoral approach (preferred approach, as with the Edwards' valves) or subclavian/axillary approach in patients who are not good candidates for transfemoral approach.

A smaller 23-mm device for annulus size 18-20 mm is under evaluation in Europe.

Several other self-expanding valves are available in Europe but have not been assessed as part of an FDA trial. These include the Sadra Lotus, Symetis and JenaValve prostheses.

Current status

Both Edwards valves (Edwards-SAPIEN and Sapien-XT) and Medtronic CoreValve are commercially available in Europe.

Sapien-XT valves and Medtronic CoreValve are currently being evaluated for safety and efficacy in randomized, controlled clinical trials—PARTNER-2, Medtronic CoreValve U.S. Pivotal Trial; and Surgical Replacement and Transcatheter Aortic Valve Implantation (SURTAVI) trials - in the United States.

Edwards-SAPIEN valve is commercially available in the United States. However, the only FDA approved indication for commercial application of the Edwards-SAPIEN valve in the United States is in high-risk inoperable patients with severe symptomatic AS (Cohort B of the PARTNER trial).

The indication for the use of Edwards-SAPIEN valve in high-risk, but operable patients with severe symptomatic AS (Cohort A of the PARTNER trial) is currently being reviewed by the FDA.

Balloon Aortic Valvuloplasty

Balloon aortic valvuloplasty (BAV) involves inflation of a balloon across the stenotic aortic valve to decrease the severity of AS. Prior to the advent of TAVR, BAV was sparingly performed in select patients; for instance, from 1990 to 2005 at the Cleveland Clinic, a large tertiary care teaching and research facility, BAV was performed in only 90 patients, as compared to 9,000 SAVRs, with SAVR:BAV ratio of 100:1.

The reasons for these low rates of BAV stem from low clinical efficacy, described later in this chapter. In the current TAVR era, there has been an increase in the number of referrals for severe symptomatic AS, with a consequent increase in the number of BAVs being performed, either as a bridge to TAVR or for palliation of symptoms in patients ineligible for TAVR. BAV is currently indicated in the following situations:

BAV alone does not alter the natural course of AS and mortality rates remain high after BAV in the absence of definitive therapy with TAVR or SAVR, with 1-year mortality rates ranging from 30-50%. On the contrary, Kapadia et al and Ben Dor et al reported that BAV, when performed as a bridge to TAVR or SAVR, is associated with superior survival, compared to BAV alone. There is an immediate improvement in: aortic valve area; mean, peak, and peak-peak pressure gradients; andsymptoms and New York Heart Association (NYHA) class in the majority of patients undergoing BAV.

These changes are transient and valve restenosis with recurrence of symptoms is generally noted within 6-12 months, unless BAV is followed by more definitive therapy with TAVR or SAVR. BAV results in fractures in the aortic valve calcium deposits with consequent increase in leaflet mobility, increase in valve area and improvement in hemodynamics. The transient duration of the beneficial effects of BAV is related to the pathology of AS. As fibrous tissue regrows over the fractured calcium deposits, the leaflets are made immobile again with a consequent recurrence of symptoms.

BAV does carry a risk of complications. In a series of 262 patients undergoing 301 BAVs from 2000-2009 representing contemporary real world practice in TAVR centers, serious adverse events occurred in 15.6% of the patients, including intraprocedural deaths (1.6%), strokes (1.99%), coronary occlusion (0.66%), severe aortic regurgitation (1.3%), resuscitation/cardioversion (1.6%), tamponade (0.33%), permanent pacemaker (0.99%), vascular complications (6.9%), a greater than 50% rise in creatinine (11.3%) and need for hemodialysis (0.99%). In conclusion, BAV is a high-risk procedure that should be performed by skilled operators in high-volume centers and only for specific indications, as outlined above.

Indications and patient selection

TAVR is currently limited to patients with severe symptomatic AS who are deemed to be at increased surgical risk due to the presence of comorbidities. For the purposes of TAVR in the U.S., severe AS is stringently defined as echocardiographically determined mean gradient greater than 40 mm Hg or jet velocity greater than 4.0 m/s or an aortic valve area less than 0.8 cm2 (or aortic valve area index less than 0.5 cm2/m2).

Since TAVR is currently limited to high-risk patients with severe symptomatic AS (except in clinical trials), determination of surgical risk to evaluate patient eligibility for TAVR is the first step in evaluating a patient for the procedure. This is done in two ways: (1) surgical scoring systems and (2) evaluation of adverse clinical and anatomic factors not included in the surgical risk calculation algorithms.

Surgical risk scoring systems

Currently, there is no TAVR risk score to determine the procedural risk for TAVR. In the absence of a dedicated TAVR risk score, surgical risk scoring systems are used to determine baseline surgical risk.

Society of Thoracic Surgeons (STS) score and EuroScore are the two principal scoring systems used to determine surgical risk in patients being evaluated for TAVR.

STS score is currently the most commonly used surgical risk scoring system used to assess surgical risk in TAVR patients and is the scoring system used to determine the surgical risk in the PARTNER (Edwards valve) and Medtronic CoreValve trials.

Clinical and anatomic factors not included in the surgical risk scoring systems

In addition to the surgical scoring systems, there are certain clinical and anatomic factors that are known to be associated with increased surgical risk but are not included in the surgical score risk algorithms. These factors include, but may not be limited to, the presence of porcelain aorta, oxygen-dependent chronic obstructive lung disease, excessive damage due to chest irradiation, chest wall deformity, vein/arterial grafts being too-close to the sternum, and frailty. The presence of these factors is determined by expert surgical opinion and may result in the patient being considered inoperable or high risk despite a low STS score.

Patients eligible for TAVR

Based on the predicted surgical risk as assessed by the STS score or clinical and anatomic risk factors, high-risk patients undergoing evaluation for TAVR can be divided into the following groups:

This group included patients with the presence of clinical or anatomic comorbidities that would confer a predicted probability of 50% or higher of either 30-day mortality or a serious irreversible condition. Some of the factors that impart surgical inoperability, despite a low STS score due to their noninclusion in the STS risk algorithm have been mentioned above.

Supportive data: TAVR results in superior 30-day, 1-year, and 2-year survival rates, compared to standard therapy in this patient population (Figure 2) and thus is currently the standard of care for inoperable patients with severe symptomatic AS.

Patients with an STS score 10% or higher or with the presence of comorbidities that would confer a predicted probability of 15% or higher of 30-day mortality are considered high-risk but operable.

Supportive data: TAVR results in similar 30-day, 1-year, and 2-year survival rates, compared to standard therapy in this patient population (Figure 3), and thus is a reasonable alternative to SAVR in high-risk but operable patients with severe symptomatic AS.

Patients considered too high risk even for TAVR (derived from post-hoc analysis of the PARTNER trial):

Commonly referred to as “cohort C.”

Although TAVR may be technically feasible in certain high-risk patients, it may not result in a clinically meaningful outcome due to the presence of comorbidities.

Post-hoc analysis of the PARTNER trial revealed that patients with STS score of 15% or higher (deemed to be at the highest surgical risk) did not derive a survival benefit from TAVR, when compared with standard therapy and thus represent a subset of patients who may be too sick to derive any benefit from the procedure (Figure 4). Thus an STS score of 15% or more may reasonably be considered as an exclusion criterion for TAVR.

Newer indications for TAVR (currently being evaluated):

Besides the above-mentioned high-risk patient populations, TAVR is currently being evaluated in prospective registries/clinical trials in patients of intermediate or greater surgical risk (PARTNER 2 STS score of 4% or more or a SURTAVI trial STS score of 4-10%) and failing degenerative bioprosthetic aortic valve disease (valve-in-valve implantation).

Figure 3.

Anatomic, Computed Tomography, and Echocardiographic Correlations of the Aortic Root. In the trifoliate aortic valve, the aortic root and its 3 leaflets form a complex 3-dimensional structure (top panel, adapted from H. Gray. Anatomy of the Human Body. Philadelphia, PA: Lea & Febiger, 1918), which is incompletely appreciated by conventional 2-dimensional echocardiographic imaging (bottom panel, intra-procedural transesophageal echocardiography [TEE]). Leaflet hinge points seen on 2-dimensional images (bottom panel) represent the interface of the leaflet and the left ventricular wall at either the nadir of the leaflet (asterisks) or at a point (white circle) that is a highly variable distance (z) above the basal plane (top, middle, and bottom panels). The TEE beam (blue triangle) represents a linear beam that images the aortic annulus posteriorly from the perspective of the left atrium. Often this cuts the basal plane obliquely, but even when through the center of the basal ring, it is impossible to determine the relationship of this cut to the true major and minor axis of the aortic basal ring (center panel). Jilaihawi et al. J Am Coll Cardiol 2012;59:1275-86. Reproduced with permission.

Due to the limited information available on the safety and efficacy of TAVR in these subgroups of patients, TAVR is not currently recommended in the United States for clinical application in these patients, outside of the setting of clinical trials or research protocols.

Contraindications

In addition to STS score of 15% or higher being a reasonable exclusion criteria for TAVR based on the results of the PARTNER trial (Cohort B), the following set of exclusion criteria, with a few modifications, are currently used in the clinical trial evaluating TAVR and are routinely followed in clinical practice.

Evidence of an acute myocardial infarction 1 month or less before the intended treatment (defined as Q wave MI, or non-Q wave MI with total CK elevation ≥ twice normal in the presence of CK-MB elevation and/or troponin level elevation [WHO definition]).

Life expectancy less than 12 months due to noncardiac comorbid conditions.

Currently participating in an investigational drug or another device study.

Active bacterial endocarditis or other active infections.

Relative exclusion criteria

With increasing experience, availability of more transcatheter heart valve sizes, lower profile delivery systems, and ongoing clinical trials, some of the exclusion criteria used in the PARTNER trial are changing, as mentioned below.

Congenital unicuspid or congenital bicuspid aortic valve, or noncalcified aortic valve. However, there are a few reports of successful deployment of transcatheter heart valves in patients with bicuspid aortic valves; thus this criterion is expected to change in future.

Preexisting prosthetic heart valve in any position, prosthetic ring, severe mitral annular calcification or severe (> 3+) mitral regurgitation. This exclusion criterion is also expected to evolve with time, since valve-in-valve implantation of transcatheter heart valves in failing degenerative bioprosthetic aortic valve stenosis is currently being evaluated in clinical trials/registries.

Untreated clinically significant coronary artery disease requiring revascularization. Although this was an exclusion criteria in the PARTNER trial; concomitant revascularization and TAVR is currently being evaluated in clinical trials. Moreover, few groups have reported little impact from coronary artery disease on outcomes after TAVR.

Native aortic annulus size under 18 mm or over 25 mm as measured by echocardiogram. The availability of 20 mm and 29 mm Sapien XT has enabled TAVR to be performed in patients with annulus size under 18 mm or over 25 mm, as measured by 2D TEE.

Significant abdominal or thoracic aorta disease, including aneurysm (defined as maximal luminal diameter of 5 cm or greater), marked tortuosity (hyperacute bend), aortic arch atheroma (especially if thick [> 5mm], protruding or ulcerated), narrowing of the abdominal aorta (especially with calcification and surface irregularities), or severe “unfolding” and tortuosity of the thoracic aorta. In these patients, TAVR can be performed through alternate vascular access, including transapical, trans-subclavian/trans-axillary or even direct transaortic approach, depending on the availability of expertise and technology.

Iliofemoral vessel characteristics that would preclude safe placement of an 18 Fr, 22 Fr, or 24 Fr introducer sheath, such as severe calcification, severe tortuosity, or vessel size diameter under 6 mm for a 18 Fr sheath, under 7 mm for a 22 Fr sheath, or under 8 mm for a 24 Fr sheath. TAVR can be performed through alternate access routes in patients with unsuitable iliofemoral arterial diameters.

Bulky calcified aortic valve leaflets in close proximity to coronary ostia. TAVR can be carefully attempted in these patients by experienced operators, with careful preemptive securing of the left main coronary artery ostium with a coronary guidewire for emergent bailout angioplasty and stenting.

Details of how the procedure is performed

Preprocedural screening

Once a patient is deemed appropriate for TAVR based on baseline surgical risk and absence of exclusion criteria for TAVR, detailed assessment of (1) the aortic root complex and (2) vascular anatomy is performed with multimodality imaging, including conventional angiography, transthoracic echocardiography (TTE), multislice computed tomography (MSCT) and/or transesophageal echocardiography (TEE) to determine the safety and feasibility of the procedure, determine the vascular access site with minimum risk of procedural complications, anticipate complications/difficulties that may arise during the procedure and prepare for contingency measures to manage complications should they occur.

Assessment of the aortic root complex

TTE is the initial imaging modality employed to evaluate the aortic root complex. However, the aortic root is a 3-dimensional structure and a complete understanding of the anatomy of the aortic root and its relationship to the surrounding structures is essential for procedural success. Moreover, a 2-dimensional imaging modality like TTE assumes the aortic annulus is a circular structure, whereas in reality, the aortic annulus is noncircular and more oval in shape, and thus determination of the size of the aortic annulus with 2D TTE or even 2D TEE is often inaccurate and associated with increased risk of paravalvular aortic regurgitation (AR). Due to the complex anatomy of the aortic root, it is better assessed with a 3-dimensional imaging modality, such as MSCT or 3D TEE (Figure 5).

Most TAVR centers have started relying heavily on MSCT for preprocedural evaluation of patients to determine the anatomy of the aortic root. In addition to assessing the anatomic characteristics of the aortic root, CT enables predicting the appropriate fluoroscopic projection for valve positioning, which is expected to reduce the risk of malpositioning. Additionally, the use of a MSCT-guided annular sizing approach results in reduction in the incidence of significant paravalvular AR after TAVR.

The assessment of aortic root involves the following:

Confirmation of the diagnosis of severe symptomatic AS. Usually, TTE can reliably assess the aortic valve area and gradients. However, TTE can be less accurate in determining true valve area in patients with depressed ejection fraction, with consequent low-flow low-gradient aortic stenosis. In such situations, the diagnosis of severe aortic stenosis is confirmed with dobutamine stress echocardiography to differentiate between true AS versus pseudo-AS. The aortic valve area remains unchanged in true aortic stenosis, while the aortic valve area is expected to increase in pseudoaortic stenosis.

Determination of the number of valve cusps to confirm a tricuspid aortic valve can occasionally be challenging with severely calcified valve leaflets.

Assessment of the maximum and minimum diameters of the aortic annulus to determine the appropriate valve size. The transcatheter aortic valves are generally oversized by 2-5 mm relative to 2D TEE measurements of aortic annulus diameter to ensure adequate anchorage and sealing of the prosthetic valve to the aortic root. Accurate determination of the annular diameter is the most crucial step for optimal valve sizing, since valve undersizing can result in valve embolization and paravalvular AR and aggressive valve oversizing can result in root rupture, and potentially central aortic regurgitation and conduction abnormalities.

Determination of the dimensions of the left ventricular outflow tract, sinuses of Valsalva sinotubular junction, and ascending aorta to determine the feasibility of the procedure and the risk of complications, including aortic root rupture and coronary occlusion.

Determination of the angulation of the ascending aorta to the aortic annulus. In case of horizontal aorta or vertical annular plane, positioning the prosthetic valve across the aortic valve by the transfemoral route can be challenging, requiring the use of buddy-balloon technique or alternative vascular access site. Left subclavian approach may be preferable in appropriately selected patients with horizontal aorta undergoing Medtronic CoreValve implantation.

Determination of the height of the coronary ostia from the aortic annulus. Low lying coronary ostium (<10 mm from the aortic annulus) is considered a risk factor for coronary compromise after TAVR.

Determination of the degree of calcification of the ascending aorta and the aortic root. Excessive calcification of the ascending aorta or the aortic root may be associated with increased risk of aortic root rupture, embolic stroke, coronary compromise and difficult valve positioning, thus necessitating transapical approach as an alternative vascular access approach.

Assessment of the vascular anatomy

Routine use of CT angiogram of the chest, abdomen and pelvis with iliac runoff to evaluate the tortuosity, calcification, and diameter of the aorta and iliofemoral vessels is recommended, since relying solely on peripheral angiography can miss certain high-risk features, including eccentric plaques or circumferential calcification or excessive anteroposterior tortuosity in the vasculature. The US PARTNER trial and US Medtronic CoreValve study committees have actually mandated routine use of CT angiogram for the assessment of the peripheral vasculature.

Due to the large size of the valve delivery systems, minimal vessel diameters of 6 mm, 7 mm and 8 mm are required for 18 Fr, 22 Fr and 24 Fr delivery systems, respectively. These are actually the respective internal dimensions of the introducer catheters, the external dimensions are inevitably even larger, measuring 21 Fr, 25 Fr and 28 Fr, respectively.

In the absence of CT assessment of iliofemoral vessels, conventional angiography can be used to predict the risk of major vascular complications in these patients by calculating sheath to femoral artery ratio (SFAR), defined as the ratio of the sheath outer diameter (in millimeters) to the minimal femoral artery diameter (in millimeters).This is a validated predictor of vascular complication.

In cases of tortuous iliofemoral vessels, the ability to straighten the tortuous vessel with a stiff wire can confirm the feasibility of the transfemoral approach.

The transfemoral route, being the least invasive approach, is the primary approach preferred by most operators. Alternative vascular access sites, including transapical and trans-subclavian approaches for Edwards and Medtronic CoreValve respectively, should be considered in case of excessive vessel tortuosity, calcification or inadequate vessel diameter. In rare situations, the procedure can be performed by the trans-iliac/abdominal aorta approach with a retroperitoneal surgical access or conduit. The direct thoracic aortic approach for Sapien and XT or transaxillary approach for the Medtronic CoreValve are exciting new alternatives in this setting that have shown promising early results.

Procedure details: Transfemoral TAVR

Vascular access

Transfemoral vascular access and closure can be achieved in 3 ways: (1) Open surgical access and closure; (2) percutaneous access and closure; and (3) percutaneous access with open surgical closure.

Vascular access and closure was initially achieved by surgical cut-down of the femoral artery under general anesthesia due to the large profiles of the early generation valves. With the newer generation lower profile 18 Fr sheaths, TAVR can be performed with percutaneous access and closure of the femoral artery in a truly percutaneous manner. Surgical cutdown is performed only in situations where difficult vascular access is anticipated; for instance, in the presence of heavy calcification at the access site or the presence of vascular grafts close to the vascular access site.

The femoral artery should be punctured below the inguinal ligament (i.e., between the origin of the inferior epigastric artery) and above the femoral artery bifurcation in the anterior wall to enable safe deployment of the suture-mediated percutaneous closure devices and endovascular repair of vascular complications that may arise during the procedure. We routinely use a micropuncture kit to obtain access at the level of femoral head under fluoroscopic guidance and confirm the level of arterial puncture with iliofemoral run-off prior to inserting the delivery sheath. Other groups have reported advancing a guidewire or pigtail catheter as landmarks by the crossover technique under fluoroscopy from the contralateral side to secure access at the desired level through the anterior wall of the femoral artery.

In cases of heavy calcification at the femoral artery access site, ultrasound guidance is recommended for vascular access to avoid puncture on a calcified plaque, which could potentially result in failure of vessel closure.

Sometimes, the transfemoral approach is feasible based on iliofemoral anatomy; however, difficulty advancing the vascular sheath through the iliofemoral vessels is anticipated due to excessive calcification, vessel tortuosity or borderline vessel diameter. The transfemoral approach can be carefully attempted in these situations by an experienced operator; however, any difficulty advancing the sheath through the iliofemoral vasculature should prompt the operator to reevaluate the situation and consider alternate access options.

Recently, expandable sheaths have been introduced to facilitate vascular access for TAVR. Solopath Vascular Sheath (Onset Medical Corporation, Irvine, CA) (Figure 3) is one such balloon expandable vascular sheath with an outer diameter of 13 Fr in the distal 25 cm, which could facilitate safer vessel entry in patients with significant atherosclerotic disease of the iliofemoral vessels. After advancing the sheath through the iliofemoral vessels, the distal portion of the sheath can be expanded to an inner diameter of 19 Fr diameter (outer diameter 22 Fr) by connecting a standard percutaneous transluminal coronary angioplasty (PTCA) balloon inflation device to the sheath-integrated balloon, facilitating passage of the 18 Fr Medtronic CoreValve delivery system through the sheath.

The manufacturer has developed collapsible introducers to reduce the likelihood of introducer-vessel adherence, which can result in vascular avulsion when the introducer is retracted. Similarly, expandable sheaths have been developed for the balloon-expandable Sapien-XT valve (eSHEATH, Edwards Lifesciences Corporation, Irvine, CA), currently available in three sizes: 16 Fr for 20-mm and 23-mm valves, 18 F for 26-mm valves and 20 Fr for 29-mm valves.

After obtaining vascular access at the desired site, the vascular sheath is placed in the abdominal aorta prior to advancing the valve delivery system.

Valve positioning and deployment

Valve positioning and deployment of the Edwards valve is performed with real-time fluoroscopic and TEE guidance while Medtronic CoreValve positioning can be performed without TEE guidance. Prior to valve deployment, balloon valvuloplasty is performed for both Edwards and Medtronic CoreValve.

It is essential to obtain the fluoroscopic projection with the three coronary sinuses aligned along a straight line to ensure optimal valve positioning. Once optimal fluoroscopic projection is obtained, the valve is positioned to cover the aortic annulus, followed by, in the case of the Edwards valve, deployment under rapid right ventricular (RV) pacing to minimize cardiac output and the risk of valve embolization during valve deployment. RV pacing is not necessary for Medtronic CoreValve implantation.

Postdeployment, optimal valve position is routinely confirmed with aortic root angiogram and TEE, whereas valve function is determined by TEE and normalization of hemodynamics between the left ventricle (LV) and the aorta.

In the presence of horizontal or unfolded aorta or excessive calcification of the aortic root, advancing the prosthetic valve across the native valve can be challenging. In such situations, advancing a second wire across the aortic valve from the contralateral femoral artery, followed by inflation of a buddy balloon over this wire or advancement of an angioplasty catheter, as a buddy catheter can provide additional anchorage and support to guide the prosthetic valve apparatus across the native valve.

In the case of the Medtronic CoreValve, a gooseneck snare loaded onto the delivery catheter over the protective sheath of the Medtronic CoreValve has in some challenging cases facilitated the passage of the delivery catheter across the native aortic valve by providing continuous traction to flex the delivery catheter as it is being advanced.

Once deployed, the balloon-expandable Edwards valve cannot be repositioned; however, a Medtronic CoreValve with a low implantation can be pulled back slowly to optimize positioning when partially released or even after full deployment, with a snare. If the Medtronic CoreValve is positioned supraannularly prior to complete release (also known as “dislocation”), the partially deployed prosthesis can be retrieved through the introducer sheath, followed by de novo placement of the prosthetic valve. However, these maneuvers are not without risks, since the friction caused by the calcified and diseased aortic wall and the angulation of the ascending aorta and aortic arch may cause vascular injury and embolic stroke.

Closure of the vascular access

There is an increasing trend towards percutaneous closure of the vascular access site with preclosure devices resulting from the availability of newer lower profile valve delivery sheaths. Percutaneous closure offers several potential advantages over open surgical closure, including the use of local anesthesia, early patient mobilization, improved patient comfort, decreased time to hemostasis and reduced length of hospital stay. Surgical closure is performed only in case of any unforeseen procedure-related or percutaneous closure device-related complications; for instance, failure to close the vascular access site with a percutaneous closure device.

Percutaneous closure is usually performed to facilitate successful closure of the access site. Either a 10 Fr Prostar or two Perclose Proglide closure devices orientated orthogonally (Abbot Vascular Inc., Santa Clara, CA) can be used for percutaneous closure of the vascular access site.

Percutaneous closure is usually performed in conjunction with the crossover balloon technique (CBOT), which involves, after valve deployment, withdrawing the delivery sheath into the common iliac artery, following which a guidewire from the contralateral femoral artery is inserted into the delivery sheath in a crossover fashion. A crossover balloon appropriately sized to the common iliac artery (e.g. Fox Plus PTA Catheter, Abbott Vascular, Santa Clara, CA) is then advanced over the wire into the valve delivery sheath, which is slowly withdrawn while performing intermittent contrast injections through the balloon’s central lumen to rule out any vascular injury.

When the valve delivery sheath is close to the access site and ready to be pulled out, the crossover balloon may be inflated in the iliac or proximal common femoral artery to offer a bloodless field while the preclosure device sutures are being deployed. The use of CBOT for percutaneous closure of the access site offers the additional advantage of placing the wire in the true lumen in case of iliac or common femoral dissection. In the event of catastrophic vascular complication, it also allows tamponading the injured site while a definitive treatment strategy is being decided. It can offer a bloodless field for vascular repair in the setting of major vascular complication, the ability to tamponade suboptimal closure and to dilate access site stenosis that can arise following closure.

Alternate vascular approaches

Due to its least invasive nature, the transfemoral approach is the most preferred approach for TAVR with either Edwards or Medtronic CoreValve implantation. However, transfemoral approach is not feasible in certain situations, thus necessitating the use of alternate vascular approaches in such circumstances that are described below.

Transapical (TA) approach

The TA approach is employed for the balloon-expandable Edwards valve TAVR in patients with unsuitable vascular anatomy for transfemoral TAVR. Approximately 30-40% of patients evaluated for TAVR are not suitable candidates for the transfemoral approach and thus the TA approach is a reasonable alternative in these patients.

The procedure involves a left lateral thoracotomy followed by direct puncture of the left ventricular apex and insertion of the valve delivery sheath into the left ventricle. Valve positioning and deployment is performed similarly to transfemoral aortic valve replacement.

There has been conflicting data regarding the outcomes after TA-TAVR versus TF-TAVR, with some reports indicating worse outcomes with transapical TAVR while other studies report similar outcomes between TA and TF-TAVR. This discrepant data appears attributable to heterogeneity of the outcomes of transapical procedures between centers.

Subclavian approach

In patients who are not candidates for TF-TAVR for Medtronic CoreValve, an axillary/subclavian approach can be employed forTAVR with Medtronic CoreValve. This involves surgical cutdown of the axillary artery, followed by insertion of the vascular sheath. Valve positioning and deployment is performed similar to transfemoral aortic valve replacement. This approach has been used in up to 20% of patients in large registries of Medtronic CoreValve implantation.

Transaortic approach

The transaortic (both Edwards and Medtronic CoreValve) approach is another alternative vascular access that can be used in patients who are not suitable candidates for TF, TA or axillary/subclavian approach. It requires a small paramedian sternotomy but, like the other approaches, can be performed without the need for cardiopulmonary bypass.

Type of anesthesia

TAVR by the transapical, subclavian or transaortic route is performed under general anesthesia. There is ongoing debate about the optimal type of anesthesia for transfemoral TAVR. Transfemoral TAVR has traditionally been performed under general anesthesia. A few groups have recently reported the safety and feasibility of performing transfemoral TAVR under local anesthesia and conscious sedation. Mottloch et al and Bergmann et al showed the use of local anesthesia and conscious sedation, in comparison to general anesthesia, to be associated with a decrease in procedural time, length of ICU stay and need for inotropic support; however, it did not have an impact on the duration of hospital stay, continuous neurologic monitoring, early detection of strokes, earlier mobilization of patients or clinical outcomes.

Moreover, the rates of conversion to general anesthesia, mostly due to procedure-related complications, have been unusually high in these series, ranging from 10-17%. With continued improvement in technology, availability of lower profile delivery systems, increasing operator experience and improved patient screening, the incidence of TAVR-related complications has been decreasing and the rates of conversion to general anesthesia due to procedure-related complications are expected to decrease in future.

In comparison to local anesthesia and conscious sedation, general anesthesia and intubation provide a more controlled environment for performing the procedure, enable real-time TEE guidance throughout the procedure and facilitate immediate management of any unforeseen complications that may occur during the procedure. TEE is an integral part of the TAVR procedure and plays a critical role in determining the annular size, early detection of periprocedural complications and confirmation of optimal valve function immediately after deployment.

Although TEE can be performed under local anesthesia and conscious sedation, it is best performed under general anesthesia in high-risk patients undergoing TAVR. Patients in cardiogenic shock and patients unable to lie flat for the procedure cannot undergo TAVR under local anesthesia with moderate sedation.

There are pros and cons of either type of anesthesia and the choice of the type of anesthesia depends on the availability of local expertise, operator experience and comfort, the incidence of complications and patient factors. While general anesthesia continues to be the anesthesia of choice in most centers, transfemoral TAVR can be performed under local anesthesia and conscious sedation in carefully selected patients, in the appropriate setting in the presence of a cardiac anesthesiologist with the capability to convert to general anesthesia immediately, if needed.

Outcomes (applies only to therapeutic procedures)

TAVR offers significant and sustained benefit in terms of survival, quality of life, NYHA class, rate of repeat hospitalizations and hemodynamic performance of the valve, without any significant deterioration over time, at least up to 2-year follow-up, as discussed below.

Some of the important determinants of survival following TAVR include logistic EuroScore, STS Score, pulmonary hypertension, coronary artery disease, operator experience, transapical access, and acute kidney injury. The determinants of survival following TAVR reported in different studies have been summarized in Table I.

Predictors of outcomes after transcatheter aortic valve replacement

There are some conflicting reports on the effect of the presence of coronary artery disease (CAD) on outcomes after TAVR, with some studies reporting higher 30-day mortality rates among patients with CAD, while other studies have reported similar survival rates in patients with or without CAD.

Hemodynamics

TAVR results in a significant and sustained improvement in hemodynamics, with an improvement in aortic valve area, transvalvular gradients and LV function. Studies have consistently reported an increase in aortic valve area and a decrease in transvalvular pressure gradients, along with a corresponding improvement in NYHA functional class following TAVR.

The clinical benefits were sustained at 2-year follow-up in the randomized controlled PARTNER trial (Figure 4) and beyond 3 years in the registries. A few studies have reported additional objective measures that support the clinical and hemodynamic benefit of TAVR, including a decrease in left ventricular mass and the left ventricular mass index, improvement in coronary blood flow and improvement in mitral regurgitation. An example depicting immediate improvement in hemodynamics after deployment of the valve is presented in Figure 5.

Quality of life

TAVR results in a significant improvement in the quality of life, compared to standard therapy in inoperable patients with severe symptomatic AS. In high-risk operable patients with severe symptomatic AS, TAVR results in a significant improvement in the quality of life that is noninferior in comparison to SAVR.

Published reports on quality of life are confounded by the heterogeneity in the scoring systems used. Within this limitation, all studies evaluating the effect of TAVR on the quality of life have consistently reported an improvement.

Complications and their management

Some of the important common complications associated with TAVR include stroke, vascular complications, AR, and conduction abnormalities.

Stroke

TAVR results in increased stroke rates compared to either standard therapy or SAVR.

Figure 6.

Effect on transcatheter aortic valve replacement (TAVR) on stroke in patients with severe symptomatic aortic stenosis in the PARTNER trial. A. TAVR results in increased stroke rates compared to surgical aortic valve replacement, in high-risk operable patients. Kodali S. et al. N Engl J Med 2012;366:1686-95. Reproduced with permission. B. In high-risk inoperable patients, TAVR results in increased stroke rates compared to standard therapy. In the first 30 days after TAVR, the increased number of strokes are primarily ischemic in etiology. From 30 days to 1 year after TAVR, the excess strokes in the TAVR group are hemorrhagic strokes, with the rates of ischemic strokes similar between TAVR and standard therapy. Makkar R. et al. N Engl J Med 2012;366:696-704. Reproduced with permission.

In inoperable patients with severe symptomatic AS, stroke rates are higher with TAVR, in comparison to standard therapy at 30 days (6.7% vs. 1.7%), 1 year (11.2% vs. 5.5%), and 2 years (13.8% vs. 5.5%) (
Figure 6 [B]). Moreover, clinically silent perfusion defects in the brain are noted on diffusion-weighted MRI imaging in approximately 70-80% of patients following TAVR. Such perfusion deficits after surgical AVR are noted in up to 30% to 40% of the patients.

The occurrence of stroke after TAVR has been associated with increased mortality in the randomized controlled PARTNER trial as well as in nonrandomized registries.

Despite the increased stroke rates associated with TAVR, the composite endpoint of death or stroke was superior to standard therapy in the inoperable cohort and noninferior to SAVR in the high-risk operable cohort of the PARTNER trial.

There is a paucity of literature on the predictors of stroke after TAVR. In a post-hoc analysis of the PARTNER trial population evaluating the predictors of stroke, TAVR and a smaller aortic valve area index (aortic valve area normalized to body surface area) were associated with an increased risk of early stroke, whereas patient-related factors ("non-TF candidate, "history of recent stroke or transient ischemic attack, and advanced functional disability) were associated with increased risk of late stroke.

Although it remains theoretically possible that the transapical insertion technique will reduce the stroke rate by decreasing aortic manipulation, clinical data has not consistently supported this hypothesis.

The vast majority of strokes associated with TAVR occur periprocedurally, and although embolic phenomena may occur during passage of the sheath through the calcified aortic arch, balloon aortic valvuloplasty and rapid pacing, transcranial Doppler data suggests that they occur most commonly during valve deployment.

Numerous embolic protection devices, for instance, the SMT Embolic Deflection Device (SMT Medical Technologies, Herzliya, Israel), the Embrella Embolic Deflector System (Embrella Cardiovascular, Inc., Wayne, PA) and the Claret Dual Filter System (Claret Medical, SantaRosa, CA) are currently being evaluated for their ability to decrease the risk of these periprocedural strokes.

Figure 7.

Transcatheter aortic valve replacement related vascular complications. A and B. External iliac dissection (A) managed with endovascular graft placement (B). C. Valve-in-valve implantation for severe aortic regurgitation noted after deployment of the first valve D. Embolization of the Edwards-SAPIEN valve to the descending aorta, with implantation of a second valve in the aortic annulus.

Complications specific to the transapical route include apical pseudoaneurysm formation and LV apical bleeding. There is a wide variation in the range of major vascular complications after TAVR due to the significant heterogeneity in the definition of the major vascular complications used in different studies. The standardized definitions of major and minor vascular complications have been defined by the Valve Academic Research Consortium (VARC).

The rates of VARC-defined major vascular complications range from 5.0% to 23.3%, with pooled rates of 11.9% (95% CI 8.6% to 16.4%); VARC-defined minor vascular complications range from 5.6% to 28.3%, with pooled rates of 9.7% (95% CI 6.7% to 14.0%). The high rate of vascular complications associated with this procedure reflects the large diameter of the valve delivery system, 22/24 Fr for the balloon-expandable Edwards valve and 18 Fr for the Sapien-XT and 3rd generation self-expanding Medtronic-CoreValve.

Technological advances have focused on the development of lower profile arterial sheaths; improved valve delivery systems; newer generation, potentially retrievable valves; and greater efficiency in the application of percutaneous vascular closure techniques. All of these factors have the potential to decrease the incidence of vascular complications.

As mentioned previously, the availability of expandable sheaths and collapsing sheaths is expected to further decrease the incidence of vascular complications, which typically become evident as the sheath is being withdrawn, since the sheath usually exerts a tamponade effect on the injury site as long as it is in the vessel.

The vascular complication may sometimes be identified at the beginning of the procedure while advancing the sheath, without any hemodynamic consequences or increased bleeding. It may be reasonable to continue the procedure in these situations and manage the vascular complication after valve deployment while the sheath is being pulled out, since patients with untreated severe symptomatic aortic stenosis may not have enough cardiopulmonary physiologic reserve to tolerate hemodynamic collapse.

In the event of vessel rupture, perforation or failure of apreclosure device resulting in uncontrolled bleeding or hemodynamic compromise, the bleeding can be controlled by inflating the percutaneous angioplasty balloon in the proximal vessel or at the site of injury, occluding flow, while preparing for definitive management.

Depending on the location and severity of the complication and the clinical condition of the patient, the vascular complications can be managed percutaneously by endovascular repair, open surgical repair or just conservatively.

Iliac or femoral arterial avulsion is a life-threatening complication associated with the procedure and requires surgical repair. Experiencing increased resistance during sheath removal can be a sign of impending arterial avulsion and should prompt the operator to perform an iliofemoral arteriogram prior to sheath removal for early detection of the avulsion and position an occlusive balloon in the distal aorta prior to sheath removal to decrease the risk of bleeding in case of an arterial avulsion.

Iliofemoral arterial dissection may be encountered during the procedure and can be managed conservatively or percutaneously depending on the site of the injury. Arterial dissection localized to the common femoral artery can usually be managed by prolonged inflation of a standard angioplasty balloon of appropriate diameter, whereas more extensive dissection involving the external iliac artery requires endovascular repair with stent-graft placement (Figure 7 [B]). It is important to maintain the true lumen in cases of arterial dissection to facilitate successful endovascular repair of the dissection.

Arterial perforations or stenoses are usually treated percutaneously with stent placement.

Pseudoaneurysms can occur at that vascular access site and are usually diagnosed with duplex Doppler ultrasound. Percutaneous thrombin injection under ultrasound guidance is employed for the treatment of large or rapidly enlarging pseudoaneurysms, whereas small pseudoaneurysm can be managed conservatively. Endovascular repair of the pseudoaneurysm with stent-graft placement is reserved for failed pseudoaneurysm repair by thrombin injection.

Aortic regurgitation

Aortic regurgitation has been reported in 60% to 90% of the patients undergoing TAVR and is the single most common complication associated with the procedure. The frequency of paravalvular AR is much higher with TAVR than with SAVR (17.0% to 45.0%). AR is predominantly mild paravalvular AR due to incomplete apposition of the prosthesis with the annular wall, with significant (≥ moderate) regurgitation reported in around 10% of patients.

Although mild paravalvular AR was initially thought to be a benign finding, the results of the PARTNER trial reported increased mortality associated with any degree of AR, including even mild AR. This may be related to the grading of AR, since paravalvular AR is extremely difficult to quantify and is frequently underestimated.

Low valve implantation, increased aortic annular/LVOT calcification and increased angle between the left ventricular outflow tract and ascending aorta are some of the risk factors associated with increased incidence of prosthetic valve AR after Medtronic-CoreValve implantation. The predictors of prosthetic valve regurgitation after TAVR are summarized in Table I.

A “device landing zone calcium score” has been proposed as a semiquantitative measure of the degree of calcification in the aortic valve and the LV outflow tract and is significantly related to paravalvular regurgitation after TAVR.

Cover index [defined as 100 x (prosthesis diameter –TEE annulus diameter)/prosthesis diameter] is a measure of congruence between the annulus and the prosthetic valve, with low cover index being associated with significant AR after TAVR.

Several measures can be attempted in the catheterization laboratory to manage AR, depending on the cause of AR. Central AR should be reassessed after removing the stiff wire from the LV across the aortic valve over a soft guide catheter. Manipulation of the prosthetic valve leaflets with a pigtail catheter can occasionally resolve central AR due to prosthetic valve leaflet dysfunction noted on TEE. Postdilatation can be attempted for paravalvular AR due to incomplete apposition of the valve to the aortic annulus that is thought to be related to stent frame underexpansion.

In the absence of any reversible causes, valve-in-valve deployment of a second transcatheter heart valve is employed as a bailout strategy to treat significant paraprosthetic AR attributable to incomplete annular coverage due to malpositioning or attributable to central valvular AR, thereby preventing conversion to emergent open-heart surgery (Figure 7 [C]).

Conduction abnormalities

Conduction abnormalities are common after TAVR, especially after Medtronic CoreValve implantation. Left bundle branch block (LBBB) is the most commonly reported conduction abnormality following TAVR, with the rates of new LBBB ranging from 22.4-65.0% following Medtronic CoreValve implantation and 5.3-20.0% following Edwards Valve implantation.

The frequency of new permanent pacemaker implantation ranges from 9.3-48.7% following Medtronic CoreValve implantation and to 10.0% following Edwards valve implantation. In comparison, the rates of permanent pacemaker requirement after SAVR range from 3-8%.

The close proximity of the aortic valvular complex to the branching atrioventricular (AV) bundle accounts for the increased frequency of conduction abnormalities and permanent pacemaker requirement following TAVR. Trauma to the conduction system during balloon valvuloplasty and stent deployment, direct trauma by catheters and guidewires, as well as the presence of the coexistent conduction system disease in these patients have been proposed as the likely explanations for the conduction abnormalities after TAVR.

The Medtronic CoreValve has been associated with an increased frequency of conduction abnormalities and a permanent pacemaker requirement after TAVR, likely due to the greater radial force of expansion of the self-expanding valve when it is deployed adjacent to the conduction system in the left ventricular outflow tract.

In addition to technical factors, specific patient characteristics may play a role in the need for new pacemaker requirement after TAVR, as summarized in Table I.

A risk prediction model with 75% sensitivity and 100% specificity for predicting the need for permanent pacemaker following Medtronic CoreValve implantation has been proposed based on the presence of at least 1 of the following factors: LBBB with left axis deviation, IVSd >17 mm and noncoronary cusp thickness >8 mm.

Due to the association between the distance of the ventricular end of the Medtronic CoreValve from the noncoronary cusp and LBBB, implanting the prosthetic valve in a more superior position in the left ventricular outflow tract may decrease the incidence of conduction abnormalities and the need for a permanent pacemaker.

Placement of a temporary pacemaker and EKG monitoring for at least 24-48 hours to determine the need for permanent pacemaker implantation is recommended in patients who develop a new LBBB or transient AV block after the deployment of the valves.

In the absence of specific guidelines on the indications for a permanent pacemaker after TAVR, it is recommended to follow ACC/AHA/HRS general guidelines for permanent pacemaker implantation. Permanent pacemaker implantation should occur in the following settings:

Other less common complications associated with TAVR included the following:

Coronary compromise

Coronary compromise occurs in up to 0.3-1.0% of patients undergoing TAVR. Some of the risk factors associated with coronary compromise after TAVR include low-lying coronary ostia, bulky leaflets, shallow sinus of Valsalva, heavy calcification, narrow aortic annulus and preexisting left main coronary artery stents.

In patients deemed to be at increased risk of coronary compromise during TAVR due to the presence of risk factors, the left main coronary artery is secured preemptively with a coronary guidewire for emergency bailout angioplasty and stenting.

Findings suggestive of coronary compromise during the procedure include appearance of new EKG changes, impaired diastolic flow in the coronaries, or new wall motion abnormalities on TEE or hemodynamic compromise following valve deployment.

Since patients undergoing TAVR are already high-risk surgical candidates at baseline, percutaneous coronary intervention is the optimal management strategy for coronary compromise due to the prosthetic valve in these patients, with CABG being available as a backup.

The embolization of the valve can occur into the aorta or into the left ventricle.

Optimal valve sizing is critical to avoid this complication, since it ensures adequate anchoring of the prosthetic valve to the aortic annulus.

Valves embolized into the aorta can be deployed in the ascending or descending aorta, away from the take-off of the great vessels in the arch of aorta, avoiding vascular compromise.

Ventricular embolization of the valve usually requires surgical intervention with cardiopulmonary bypass to remove the embolized valve.

With the malpositioned Medtronic CoreValve, one can attempt valve repositioning of a prosthesis fully deployed too ventricular with a snare, or retrieval of a valve dislocated into the aorta, if not fully released.

A second prosthetic valve can be deployed in the aortic annulus in the same setting in most of the cases with adequate hemodynamic recovery.

Cardiac tamponade

Cardiac tamponade occurs in up to 6.7% of patients undergoing TAVR.

It often presents as sudden hemodynamic compromise during the procedure and can be identified on fluoroscopy as a radiolucent halo around the heart border or can be easily detected on periprocedural TEE.

Tamponade can occur due to perforation of the RV with the temporary pacemaker, LV with the stiff wire used to cross the aortic valve or, rarely, following aortic root rupture.

Depending on the severity and persistence of bleeding underlying tamponade, it can be managed with percutaneous drainage alone, drainage with auto-transfusion or open heart surgery under cardiopulmonary bypass.

Prosthetic valve endocarditis

Prosthetic valve endocarditis is a rare, but significant complication of TAVR, presenting as low-grade pyrexia, worsening left ventricular or prosthetic valve function, or a mitral valve aneurysm and perforation.

In the absence of specific guidelines for the prevention of prosthetic valve endocarditis following TAVR, strict endocarditis prophylaxis is recommended in these patients according to ACC/AHA guidelines for antibiotic prophylaxis in patients with valvular heart disease.

Patients undergoing TAVR are routinely administered one dose of broad spectrum antibiotic 30 minutes prior to beginning the procedure, with additional doses to cover the 24 hours postprocedure, to decrease the probability of periprocedural seeding of the prosthetic valve with bacteria.

It is speculated that suboptimal valve positioning can potentially result in an increased risk of prosthetic valve endocarditis. Low valve positioning predisposes to paravalvular regurgitation, as well as repetitive friction between the anterior mitral valve leaflet and the ventricular end of the prosthesis. High valve positioning can also result in increased prosthetic valve regurgitation. All of these clinical scenarios can predispose the prosthetic valve to endocarditis.

Other measures to prevent prosthetic valve endocarditis include aggressive treatment of infections, most common infections being urinary tract infections, pulmonary and vascular access infections, following strict aseptic techniques during the procedure, with the use of a hybrid operating room.

SAVR, with or without aortic root preservation, is the only definitive treatment for prosthetic valve endocarditis refractory to medical therapy. This carries substantial putative risk in patients who were already deemed inoperable or high risk for SAVR.

(The above two articles report the results of the high-risk but operable cohort of the landmark, randomized, controlled PARTNER trial comparing TAVR with SAVR in patients with severe symptomatic AS. This trial showed that TAVR results in a sustained, significant improvement in survival, quality of life, hemodynamics and heart failure hospitalizations, that is noninferior to SAVR, in high-risk, operable patients. This study established TAVR as an alternative to SAVR in high-risk operable patients with severe symptomatic AS.)

(This article is an expert consensus document on TAVR released by the American College of Cardiology Foundation (ACCF), American Association for Thoracic Surgery (AATS), Society for Cardiovascular Angiography and Interventions (SCAI), and the Society of Thoracic Surgeons (STS) and has been developed to examine the current state of the evidence, facilitate the integration of this technology into the treatment options for patients with severe symptomatic AS and to enable appropriate adoption of this technology.)

(The above two papers report the superiority of CT over TTE and TEE in the evaluation of aortic annulus and improvement in the degree of paravalvular aortic regurgitation by the use of cross-sectional CT for annular sizing.)

(This paper describes the crossover balloon technique that is used along with percutaneous access and closure of the vascular access sites and results in a decrease in the incidence of vascular complications.)